1. Field of the Invention
The present invention relates to an optical disk suitable for use in a high density recording, in which phase pits are formed in a transparent substrate. A material layer, whose reflection factor is changeable according to the temperature, is formed on the transparent substrate.
2. Background of the Art
A digital audio disk (i.e. compact disk) or an optical disk (e.g. video disk) is composed of an aluminum reflection film formed on a transparent substrate in which phase pits have been previously formed according to information signals, and a protecting film formed on the aluminum reflection film.
In the optical disk as described above, signals are read out or reproduced from the disk by irradiating a reading light upon the disk surface so as to detect a remarkable decrease in the quantity of the reflected light caused by the diffraction on the disk surface in which phase pits are formed.
In the conventional optical disk as described above, the resolving power in the signal reproduction can be almost determined by the wavelength .lambda. of a light source of the reproducing optical system and the numerical aperture NA of an objective lens, and the reproduction limit is determined by the spacial frequency of 2 NA/.lambda..
Therefore, in order to realize an optical disk of higher density, it is indispensable to decrease the wavelength .lambda. of the light source (e.g. semiconductor laser) of the reproducing optical system and further to increase the numerical aperture NA of the objective lens.
However, there exists a limit, as a matter of course, in improvement of the wavelength .lambda. of the light source and the numerical aperture NA of the objective lens, so that it is difficult to increase the recording density remarkably, in practice.
To overcome the above-mentioned problem, the same applicant has already proposed an optical disk which can realize a high resolving power beyond the above-mentioned limit determined by the wavelength .lambda. and the numerical aperture NA, by changing the reflection factor on the basis of change in the partial phase within a scanning spot of the reading light, as disclosed in Japanese Patent Application Nos. 2-94452 and 3-249511.
FIG. 4 is a cross-sectional view showing an example of this optical disk, in which a material layer 3 crystallizable after having been melted is formed on a flat transparent substrate 2 on which phase pits 1 are formed in accordance with information signals.
In this optical disk, when a reproducing laser light, for instance is applied onto the material layer 3, a temperature distribution is produced within the scanning spot of the reading light. Therefore, the material layer 3 is locally melted from the crystal state to reduce the reflection factor, and then returned to the original crystal state in the steady state after reading operation.
The irradiation of the reproducing laser light upon the optical disk shown in FIG. 4 will be described in further detail with reference to FIG. 5.
In FIG. 5(A), a laser spot SP is scanned in the arrow direction SC when the disk is being rotated. In the drawing, the respective phase pits 1 are arranged at the shortest possible recording period q. However, this arrangement of intervals and the pit length change of course according to the recording data.
Further, in FIG. 5(B), the abscissa indicates the position of the laser spot SP in the scanning direction SC. Under the condition that the laser spot SP is irradiated upon the optical disk as shown in FIG. 5A, the intensity of the laser spot SP distributes as shown by a dashed curve a. On the other hand, the temperature on the material layer 3 of the optical disk distributes as shown by a solid curve b, which is slightly delayed from the intensity of the laser spot SP according to the scanning speed of the laser spot SP.
Here, as described above, when the laser spot SP is scanned in the scanning direction SC as shown in FIG. 5A, the temperature at the laser spot SP on the optical disk increases gradually from the start end side along the scanning direction of the laser spot SP, and exceeds the melting point MP of the material layer 3.
At this stage, the material layer 3 changes from the initial crystal state to a melted state, so that the reflection factor drops. Therefore, there exist two regions, that is, a region P.sub.x (shown by a shaded portion in FIG. 5A) in which the reflection factor is low and therefore a phase pit 1 is not readable and another region P.sub.z in which the reflection factor is high due to the crystal state and therefore the phase pit 1 is readable.
Therefore, as shown in FIG. 5A, even if there are two phase pits 1 within the same laser spot SP, for instance, it is possible to read data from only one phase pit 1 existing in the high reflection factor region P.sub.z. Consequently, it is possible to read data with an ultra-high resolving power without being subjected to the limit determined by the wavelength .lambda. of the reading light and the numerical aperture NA of the objective lens, thus enabling a high density recording.
Further, in the above-mentioned optical disk (FAD type), the reflection factor is low when the material layer 3 is in the melted state and high when in the crystal state. However, it is also possible to construct the material layer 3 in such a way that the reflection factor is high in the melted state and low in the crystal state, according to the selection of various conditions (structure, thickness, etc.) of the material layer 3. In the optical disk (RAD type) as described above, data is readable from only the region (refer to the shaded portion in FIG. 5A) in which the material layer is in the melted state and therefore the reflection factor increases. That is, in both the RAD and FAD types, it is possible to read data with an ultra-high resolving power and therefore to realize a high density recording.
In the above-mentioned optical disk (referred to as SR disk) where the material layer 3 is formed on the transparent substrate 2 on which phase pits 1 are arranged, the spread of the heat flow in the track direction has not so far taken into account.
Therefore, in the case of the RAD type SR disk, it has been to practice to keep the track intervals separated beyond the radius of the reading optical spot, to such an extent that the track is not susceptible to change in the reflection factor of the adjacent track, in order to prevent crosstalk between two adjacent tracks due to change in the reflection factor, thus raising a problem that the track density will not be increased to realize higher density recording.